BACKGROUND: Titanium dioxide nanoparticles (TiO2NPs) are used in many commercial products, including paints, sunscreens and edible products, and can be used directly and indirectly in aquaculture. Due to the widespread application of these nanoparticles, the investigation of their possible effects on aquatic organisms is necessary. OBJECTIVES: The purpose of this study was to investigate the histopathological effects of waterborne TiO2NPs on the gills, liver and intestine of Caspian trout. METHODS: In this study, 126 fish (27.46±4.3 g) were tested in six concentrations of TiO2NPs (0, 0.01, 0.1, 1, 10 and 100 mg/L) for 4 days. At the end of the experiment, tissues were evaluated to determine the effects of tissue damage caused by TiO2NPs. RESULTS: The 96-h exposure to TiO2NPs did not cause fish mortality. Accordingly, investigated TiO2NPs are classified as "relatively non-toxic". The most important tissue changes caused by short-term exposure to TiO2NPs were hyperplasia and hypertrophy, lamellar fusion, necrosis in gills; foci of melanomacrophage deposits, aggregation of blood cell, hepatocytes with pycnotic nuclei, vacuolation, necrosis in liver; thickening of lamina propria, erosion of villi and necrosis in intestine. CONCLUSIONS: Although TiO2NPs are relatively non-toxic,  due to the tissue damage observed, these nanoparticles can be harmful to Caspian trout.

BACKGROUND: Ichthyophthiriasis induced by a freshwater teleost pathogen “Ichthyophthirius multifiliis” is one of the most important parasitic diseases with substantial economic losses to the aquaculture industry. Although malachite green, copper sulphate, formalin, and chloramine-T have been used to treat ichthyophthiriasis, there are  no chemicals that can be used safely and effectively to control this parasitic disease. As a result, application of natural, safe and strong components to prevent ichthyophthiriasis is of great importance. Objective: The aim of the present research is to determine the short-term effectiveness of tannic acid (TA) on the parasite theronts of I. multifiliis. Methods: In this study, TA as a main phenolic acid at different concentrations (0.0-7.0 mg/L) was applied to determine its acute toxicity against I. multifiliis theronts in various exposure times (1-3 h). The results were also statistically compared to the findings obtained from the control treatment and the positive control sample (15mg/L  formalin). Results: There was a significant and direct correlation between TA concentration and exposure time in order to enhance the mortality rate of I. multifiliis theronts. An increase in levels of TA and exposure time in the studied ranges can significantly intensify the mortality number (P<0.05). The used natural phenolic constituent similar to 15 mg/L formalin led to a significant reduction in number of these theronts (> 80%) at 60 min. Conclusions: Use of a standard phenolic agent such as TA at higher concentration and longer exposure time can potentially decrease the number of I. multifiliis theronts and control ichthyophthiriasis.